Buffered hydrofluoric acid, i.e., a mixture of hydrofluoric acid and ammonium fluoride solution, has been used for a wet etching solution for silicon oxide films (for example, Patent Documents 1 and 2). In a semiconductor manufacturing process, etching is performed by immersing a wafer in a chemical bath containing buffered hydrofluoric acid. However, there is a known drawback: because a chemical bath usually has an opening for immersing a wafer, the chemical composition changes over time due to the evaporation of chemical components, which significantly affects the etch rate. Accordingly, the entire amount of the chemical solution needs to be replaced over time, which poses problems in terms of process efficiency and the effective use of resources.
Paragraph 0005 in Patent Document 1 states that “when the buffered hydrofluoric acid (BHF) whose chemical composition is 0.1% HF and 40% NH4F is used, the etch rate of a thermally-oxidized film is 1.6 nm/min at 25° C. However, when such a chemical solution is left to stand for 3 days in an environment with a humidity of 40% and an ambient temperature of 25° C., the etch rate becomes 4.2 nm/min, which is approximately 2.6 times faster. Such an environment-dependent chemical solution cannot be used in a semiconductor process in the future”.
The dissociation of ammonium fluoride, ammonium ion, and hydrofluoric acid in buffered hydrofluoric acid is expressed by the following Formulae (1) to (4).NH4F→NH4++F−  (1)NH4+⇄NH3+H+(pKa=9.24)  (2)HF⇄H++F−(pKa=3.17)  (3)HF+F−⇄HF2−  (4)
When the buffered hydrofluoric acid having a chemical composition as described in Patent Document 1 (HF=0.1 mass %, NH4F=40 mass %) is left to stand, for example, in an environment with a humidity of 40% and a temperature of 25° C., ammonia evaporates according to Formula (2), and protons (H+) are released into the solution. Because hydrofluoric acid is a weak acid having a pKa of 3.17, the released H+ reacts with fluoride ion (F−) produced according to Formula (1), generating hydrogen fluoride (HF) according to Formula (3). HF further reacts with F−, generating HF2− that acts as an etching species for a silicon oxide film. Consequently, when buffered hydrofluoric acid having a chemical composition as described above is left to stand, the chemical composition changes and the etch rate of the silicon oxide film increases, and the chemical solution becomes unusable.
In addition, it is stated in paragraph 0019 of Patent Document 1 that “when the NH4F concentration is greater than 30 mass %, although the amount of evaporation of the chemical solution is small, the composition of the chemical solution changes. Consequently, it is difficult to restore the chemical solution to the original composition”.
Further, Patent Document 1 discloses a technology relating to buffered hydrofluoric acid, specifically, to one having an HF concentration of not more than 0.1 mass % and the NH4F concentration of not more than 30 mass %. In this technology, a time-dependent change in the chemical composition is reduced by supplying a composition-adjusting chemical solution to a chemical bath, thereby maintaining the predetermined chemical composition of a chemical solution in the chemical bath whose composition has changed. However, Patent Document 1 merely describes a reduction of the change in the chemical composition by controlling the environment in which the chemical solution is used. The described technology does not reduce change in the composition by improving the chemical solution itself.                Patent Document 1: Japanese Unexamined Patent Publication No. 9-22891        Patent Document 2: Japanese Unexamined Patent Publication No. 9-115875        